U.S. patent number 6,179,983 [Application Number 08/969,267] was granted by the patent office on 2001-01-30 for method and apparatus for treating surface including virtual anode.
This patent grant is currently assigned to Novellus Systems, Inc.. Invention is credited to Jonathan David Reid, Steve Taatjes.
United States Patent |
6,179,983 |
Reid , et al. |
January 30, 2001 |
Method and apparatus for treating surface including virtual
anode
Abstract
An apparatus for depositing an electrical conductive layer on
the surface of a wafer includes a virtual anode located between the
actual anode and the wafer. The virtual anode modifies the electric
current flux and plating solution flow between the actual anode and
the wafer to thereby modify the thickness profile of the deposited
electrically conductive layer on the wafer. The virtual anode can
have openings through which the electrical current flux passes. By
selectively varying the radius, length, or both, of the openings,
any desired thickness profile of the deposited electrically
conductive layer on the wafer can be readily obtained.
Inventors: |
Reid; Jonathan David (Sherwood,
OR), Taatjes; Steve (West Linn, OR) |
Assignee: |
Novellus Systems, Inc. (San
Jose, CA)
|
Family
ID: |
25515364 |
Appl.
No.: |
08/969,267 |
Filed: |
November 13, 1997 |
Current U.S.
Class: |
205/96; 204/227;
204/228.1; 204/230.2; 204/230.3; 204/242; 204/DIG.7; 205/118;
205/157 |
Current CPC
Class: |
C25D
7/123 (20130101); C25D 17/001 (20130101); C25D
17/007 (20130101); C25D 17/12 (20130101); Y10S
204/07 (20130101) |
Current International
Class: |
C25D
5/00 (20060101); C25D 7/12 (20060101); C25D
005/00 (); C25D 017/08 () |
Field of
Search: |
;205/96,118,157
;204/242,227,228,DIG.7,228.1,230.2,230.3 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Phasge; Arun S.
Attorney, Agent or Firm: Skjerven Morrill MacPherson LLP
Steuber; David E.
Parent Case Text
CROSS REFERENCE TO RELATED APPLICATION
This application is related to Patton et al., co-filed application
Ser. No. 08/969,984; Contolini et al., co-filed application Ser.
No. 08/970,120; and Reid et al., co-filed application Ser. No.
08/969,196, now abandoned all filed Nov. 13, 1997, all of which are
incorporated herein by reference in their entirety.
Claims
We claim:
1. An apparatus for treating the surface of a substrate
comprising:
a clamshell for holding said substrate;
a plating bath having a wall section;
a virtual anode having a periphery secured to said wall section,
said virtual anode having at least one opening therein; and
an anode, said virtual anode being located between said clamshell
and said anode.
2. The apparatus of claim 1 wherein said virtual anode has a
plurality of openings therein.
3. The apparatus of claim 2 wherein at least one of said plurality
of openings has a different length than at least one other of said
plurality of openings.
4. The apparatus of claim 2 wherein at least one of said plurality
of openings has a different radius than at least one other of said
plurality of openings.
5. The apparatus of claim 2 wherein at least one of said plurality
of openings has a different radius and a different length than at
least one other of said plurality of openings.
6. The apparatus of claim 1 wherein said virtual anode has a
contoured cross-section.
7. The apparatus of claim 1 wherein said virtual anode has a
stepped cross-section.
8. The apparatus of claim 1 further comprising a plating solution,
wherein said plating solution flows in said plating bath from said
anode to said clamshell through said at least one opening.
9. The apparatus of claim 8 further comprising a power supply for
generating an electric current flux between said surface of said
substrate and said anode.
10. The apparatus of claim 9 wherein said electric current flux
passes through said virtual anode.
11. The apparatus of claim 10 wherein said virtual anode has a
plurality of openings therein, a first opening of said plurality of
openings having a greater length than a second opening of said
plurality of openings, said first opening having a greater
electrical resistance to said electric current flux than said
second opening.
12. The apparatus of claim 11 wherein a greater percentage of said
electric current flux passes through said second opening than
through said first opening.
13. The apparatus of claim 10 wherein said virtual anode has a
plurality of openings therein, a first opening of said plurality of
openings having a greater radius than a second opening of said
plurality of openings, said second opening having a greater
electrical resistance to said electric current flux than said first
opening.
14. The apparatus of claim 13 wherein a greater percentage of said
electric current flux passes through said first opening than
through said second opening.
15. The apparatus of claim 1 wherein said virtual anode comprises
an electrically insulating material.
16. A method of treating a surface of a substrate comprising the
steps of:
providing a clamshell, an anode, a virtual anode, and a plating
bath containing a plating solution;
mounting said substrate in said clamshell;
placing said clamshell and said substrate in said plating solution;
and
generating an electric current flux between said surface of said
substrate and said anode, wherein said electric current flux passes
through said virtual anode, said virtual anode shaping said
electric current flux according to a distance between said virtual
anode and said substrate.
17. The method of claim 16 wherein said virtual anode has a
plurality of openings therein, wherein said electric current flux
passes through said plurality of openings and thereby through said
virtual anode.
18. The method of claim 17 wherein a first opening of said
plurality of openings has a greater cross-sectional area than a
second opening of said plurality of openings, a greater percentage
of said electric current flux passing through said first opening
than through said second opening.
19. The method of claim 18 wherein said first opening and said
second opening are cylindrical, the electric current flux through
said first opening and said second opening being directly
proportional to the square of the radius of said first opening and
said second opening.
20. The method of claim 19 further comprising the step of
generating a flow of said plating solution through said virtual
anode, wherein a greater percentage of said plating solution flow
passes through said first opening than through said second
opening.
21. The method of claim 20 wherein the plating solution flow
through said first opening and said second opening is directly
proportional to the cube of the radius of said first opening and
said second opening.
22. The method of claim 21 wherein the difference in plating
solution flow through said first opening and said second opening is
non-linear to the difference in electric current flux through said
first opening and said second opening.
23. The method of claim 22 wherein the difference in plating
solution flow through said first opening and said second opening is
greater than a difference in electric current flux through said
first opening and said second opening.
24. A method of treating a surface of a substrate comprising:
providing a clamshell an anode a virtual anode having a plurality
of openings therein, a first opening of said plurality of openings
having a greater length than a second opening of said plurality of
openings, and a plating bath containing a plating solution;
mounting said substrate in said clamshell;
placing said clamshell and said substrate in said plating solution;
and
generating an electric current flux between said surface of said
substrate and said anode, wherein said electric current flux passes
through said plurality of openings and thereby through said virtual
anode, a greater percentage of said electric current flux passing
through said second opening than through said first opening, said
virtual anode shaping said electric current flux.
25. The method of claim 24 wherein the electric current flux
through said first opening and said second opening is inversely
proportional to the length of said first opening and said second
opening.
26. The method of claim 24 further comprising the step of
generating a flow of said plating solution through said virtual
anode, wherein a greater percentage of said plating solution flow
passes through said second opening than through said first
opening.
27. The method of claim 26 wherein the plating solution flow
through said first opening and said second opening is inversely
proportional to the length of said first opening and said second
opening.
28. The method of claim 26 wherein the difference in plating
solution flow through said first opening and said second opening is
linear to the difference in electric current flux through said
first opening and said second opening.
29. A method of electroplating a metallic layer on a substrate
comprising:
immersing said substrate in an electroplating solution;
immersing an anode in said solution;
applying a positive voltage to said anode and a negative voltage to
said substrate;
interposing a virtual anode in said electroplating solution between
said anode and said substrate, said virtual anode comprising at
least a first opening and a second opening; and
causing said first opening to have a first width and a first length
and said second opening to have a second width and a second length
so as to produce a particular thickness profile of said metallic
layer, said thickness profile being determined at least in part by
said first and second widths and said first and second lengths.
30. The method of claim 29 comprising creating a flow of said
electroplating solution through said first and second openings in a
direction from said anode to said substrate.
31. An electroplating system for semiconductor wafers
comprising:
a power supply having a negative terminal and a positive
terminal;
a semiconductor wafer electrically connected to the negative
terminal;
a plating bath holding a plating solution;
an anode positioned in the plating solution and electrically
connected to the positive terminal;
a nonconductive virtual anode positioned in the plating solution
between the anode and the wafer, the virtual anode being in the
form of an annulus having a central aperture with a diameter that
is less than a diameter of the anode.
32. The electroplating system of claim 31 wherein the diameter of
the central aperture is less than a diameter of the wafer.
33. A method of electroplating a layer of metal on a semiconductor
wafer comprising:
immersing the wafer in a plating solution;
immersing an anode in the plating solution;
applying a negative voltage to the wafer and applying a positive
voltage to the anode; and
positioning a virtual anode between the anode and the wafer, the
virtual anode being in the form of an annulus having a central
aperture with a diameter less than a diameter of the wafer such
that the virtual anode functions to limit a flow of current to an
edge region of the wafer.
34. The method of claim 33 wherein the diameter of the central
aperture of the virtual anode is less than a diameter of the anode.
Description
FIELD OF INVENTION
The present invention relates generally to an apparatus for
treating the surface of a substrate and more particularly to an
apparatus for electroplating a layer on a semiconductor wafer.
BACKGROUND OF THE INVENTION
The manufacture of semiconductor devices often requires the
formation of electrical conductors on semiconductor wafers. For
example, electrically conductive leads on the wafer are often
formed by electroplating (depositing) an electrically conductive
layer such as copper on the wafer and into patterned trenches.
Electroplating involves making electrical contact with the wafer
surface upon which the electrically conductive layer is to be
deposited (hereinafter the "wafer plating surface"). Current is
then passed through a plating solution (i.e. a solution containing
ions of the element being deposited, for example a solution
containing Cu.sup.++) between an anode and the wafer plating
surface (the wafer plating surface being the cathode). This causes
an electrochemical reaction on the wafer plating surface which
results in the deposition of the electrically conductive layer.
To minimize variations in characteristics of the devices formed on
the wafer, it is important that the electrically conductive layer
be deposited uniformly (have a uniform thickness) over the wafer
plating surface. However, conventional electroplating processes
produce nonuniformity in the deposited electrically conductive
layer due to the "edge effect" described in Schuster et al., U.S.
Pat. No. 5,000,827, herein incorporated by reference in its
entirety. The edge effect is the tendency of the deposited
electrically conductive layer to be thicker near the wafer edge
than at the wafer center.
To offset the edge effect, Schuster et al. teaches non-laminar flow
of the plating solution in the region near the edge of the wafer,
i.e., teaches adjusting the flow characteristics of the plating
solution to reduce the thickness of the deposited electrically
conductive layer near the wafer edge. However, the range over which
the flow characteristics can be thus adjusted is limited and
difficult to control. Therefore, it is desirable to have a method
of offsetting the edge effect which does not rely on adjustment of
the flow characteristics of the plating solution.
Another conventional method of offsetting the edge effect is to
make use of "thieves" adjacent the wafer. By passing electrical
current between the thieves and the anode during the electroplating
process, electrically conductive material is deposited on the
thieves which otherwise would have been deposited on the wafer
plating surface near the wafer edge where the thieves are located.
This improves the uniformity of the deposited electrically
conductive layer on the wafer plating surface. However, since
electrically conductive material is deposited on the thieves, the
thieves must be removed periodically and cleaned, thus adding to
the maintenance cost and downtime of the apparatus. Further,
additional power supplies must be provided to power the thieves,
adding to the capital cost of the apparatus. Accordingly, it is
desirable to avoid the use of thieves.
SUMMARY OF THE INVENTION
In accordance with the present invention, there is provided a
"virtual" anode between the actual anode (hereinafter "the anode")
and the wafer plating surface. This virtual anode, made of an
electrically insulating material, acts to modify the electric
current flux and the plating solution flow between the anode and
the wafer plating surface in a manner which can be controlled by
the shape and location of this virtual anode. Since the thickness
of the deposited electrically conductive layer at any particular
region of the wafer plating surface is determined by the electric
current flux to the particular region, this virtual anode permits
any desired thickness profile of the deposited electrically
conductive layer.
In one embodiment, the virtual anode takes the form of a member
positioned between the anode and the wafer plating surface, this
member having at least one opening therein through which plating
solution flows. This virtual anode has the effect of regulating
both the electric current flux and the plating solution flow
between the anode and the wafer plating surface, depending upon the
shape and location of the virtual anode. The virtual anode also has
the effect of "decoupling" the electric current flux from the
plating solution flow so that the two variables may be controlled
independent of each other.
In one embodiment of the invention, the virtual anode has a
plurality of openings therein, at least one of which is of a
different cross-sectional area than at least one of the others, or
is of a different length, or both. In general, a change in the
cross-sectional area of an opening produces a greater change in the
plating solution flow than in the electric current flux through the
opening. Thus, by using openings of different cross-sectional area,
the plating solution flow can be decoupled (independently varied)
from the electric current flux through the openings. In contrast, a
change in the length of an opening produces a linear change in both
the plating solution flow and the electric current flux through the
opening.
In one particular embodiment the openings are cylindrical. In this
embodiment, the electric current through any particular opening is
inversely proportional to the length of the opening and is directly
proportional to the square of the radius of the opening. The
plating solution flow through any particular opening is also
inversely proportional to the length of the opening. However, in
contrast to the electric current flux which is directly
proportional to the square of the radius of the opening, the
plating solution flow through any particular opening is directly
proportional to the cube of the radius of the opening. Similar
relations exist for openings of other shapes. Thus, by combining
various openings of variable length and variable cross-sectional
area, electric current flux and plating solution flow to the wafer
can be controlled and, if desired, decoupled from one another. This
allows any desired thickness profile of the deposited electrically
conductive layer on the wafer plating surface to be obtained.
In a first alternate embodiment, the virtual anode is in the form
of an annulus attached to an anode cup of the anode. This virtual
anode acts as a shield to limit the amount of electric current flux
at the edge region of the wafer by forcing the electric current
flux to pass around the virtual anode, thereby reducing the
thickness of the deposited electrically conductive layer on the
wafer edge region.
In the second alternative embodiment, intended for use when it is
desired to have a relatively thick deposit on the edge region of
the wafer and a relatively thin deposit on the center region, the
virtual anode comprises a disk overlying the center of the anode.
This virtual anode effectively shields the center region of the
wafer from the electric current flux thereby reducing the thickness
of the deposited electrically conductive layer on the center
region.
BRIEF DESCRIPTION OF DRAWINGS
FIG. 1 is a diagrammatic view of an electroplating apparatus having
a virtual anode mounted therein in accordance with the present
invention;
FIG. 2 is a cross-sectional view of an electroplating apparatus and
one embodiment of a virtual anode in accordance with the present
invention;
FIG. 3 is a diagrammatic representation of the effect of a virtual
anode having variable length openings on the electric current flux
between the anode and the wafer plating surface in accordance with
the present invention;
FIG. 4 is a diagrammatic representation of the effect of a virtual
anode having variable radius openings on the electric current flux
between the anode and the wafer plating surface in accordance with
the present invention;
FIG. 5 is a cross-sectional view of an alternate embodiment of the
virtual anode in accordance with the present invention;
FIG. 6 is a cross-sectional view illustrating another embodiment of
a virtual anode which acts to shield the edge region of the wafer
in accordance with the present invention; and
FIG. 7 is an isometric view of a further embodiment of a virtual
anode which acts to shield the center region of the wafer in
accordance with the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT
FIG. 1 is a diagrammatic view of an electroplating apparatus in
accordance with the present invention. Apparatus 30 includes a
clamshell 32 mounted on a rotatable spindle 40 which provides
rotation of clamshell 32. Clamshell 32 comprises a cone 34 and a
cup 36. A clamshell of a type for use as clamshell 32 is described
in detail in Patton et al., co-filed application Ser. No.
08/969,984, identified above.
During the electroplating process, a wafer 38 preferably having an
electrically conductive seed layer thereon is mounted in cup 36.
Clamshell 32 and hence wafer 38 are then placed in a plating bath
42 containing a plating solution. The plating solution is
continually provided to plating bath 42 by a pump 44. Generally,
the plating solution flows upwards through openings in anode 62 and
around anode 62 (to be explained further in connection with FIG. 2)
toward wafer 38.
Disposed between anode 62 and wafer 38 is one embodiment of a
virtual anode 10 in accordance with this invention. The periphery
of virtual anode 10 is secured to a cylindrical wall 198 of plating
bath 42 and is positioned at a distance from wafer 38 which is
determined by the desired thickness profile of the electrically
conductive layer to be deposited on wafer 38. The general rule is
that the closer virtual anode 10 is to wafer 38, the greater the
influence virtual anode 10 has on the resulting thickness profile
of the electrically conductive layer to be deposited on wafer 38,
as will be described in more detail below. Since virtual anode 10
is secured (sealed) to wall section 198 of plating bath 42, the
plating solution flows through virtual anode 10. After flowing
through virtual anode 10, the plating solution then overflows
plating bath 42 to an overflow reservoir 56, as indicated by arrows
54. The plating solution is filtered (not shown) and returned to
pump 44 as indicated by arrow 58, completing the recirculation of
the plating solution.
A DC power supply 60 has a negative output lead 210 electrically
connected to wafer 38 through one or more slip rings, brushes and
contacts (not shown). The positive output lead 212 of power supply
60 is electrically connected to anode 62 located in plating bath
42. During use, power supply 60 biases wafer 38 to have a negative
potential relative to anode 62, causing an electrical current to
flow from anode 62 through virtual anode 10 to wafer 38. As used
herein, electrical current flows in the same direction as the net
positive ion flux and opposite the net electron flux, wherein
electric current is defined as the amount of charge flowing through
an area per unit time. This also causes an electric current flux
from anode 62 through virtual anode 10 to wafer 38, wherein
electric current flux is defined as the number of lines of forces
(field lines) through an area. This causes an electrochemical
reaction (e.g. Cu.sup.++ +2e.sup.- =Cu) on wafer 38 which results
in the deposition of the electrically conductive layer (e.g.
copper) on wafer 38. The ion concentration of the plating solution
is replenished during the plating cycle by dissolving a metal in
anode 62 which includes, for example, a metallic compound (e.g.
Cu=Cu.sup.++ +2e.sup.-), as described in detail below.
FIG. 2 is a cross-sectional view of anode 62 and virtual anode 10
in plating bath 42, plating bath 42 including cylindrical wall
section 198. Anode 62 comprises an anode cup 202, ion source
material 206, and a membrane 208. Anode cup 202 is typically an
electrically insulating material such a polyvinyl chloride (PVC).
Anode cup 202 comprises a disk shaped base section 216 having a
plurality of spaced openings 216A therein through which plating
solution flows. Anode cup 202 further comprises a cylindrical wall
section 218 integrally attached at one end (the bottom) to base
section 216.
An electrical contact and filter sheet is typically provided, as
shown in detail in the application Reid et al., Ser. No. 08/969,196
identified above, now abandoned. The contact 204 may be in the form
of an electrically conductive, relatively inert mesh such as
titanium mesh, and rests on the filter sheet which rests on base
section 216 of anode cup 202. Resting on and electrically connected
with contact 204 is ion source material 206, for example copper.
During use, ion source material 206 electrochemically dissolves
(e.g. Cu=Cu.sup.2+ +2e.sup.-), replenishing the ion concentration
of the plating solution.
Ion source material 206 is contained in an enclosure formed by
anode cup 202 and membrane 208. More particularly, membrane 208
forms a seal at its outer circumference with a second end (the top)
of wall section 218 of anode cup 202. Although allowing electrical
current to flow through, membrane 208 has a high electrical
resistance which produces a voltage drop across membrane 208 from
the lower surface to the upper surface. This advantageously
minimizes variations in the electric field from ion source material
206 as it dissolves and changes shapes.
In addition to having a porosity sufficient to allow electrical
current to flow through, membrane 208 also has a porosity
sufficient to allow plating solution to flow through membrane 208,
i.e. has a porosity sufficient to allow liquid to pass through
membrane 208. However, to prevent particulates generated by ion
source material 206 from passing through membrane 208 and
contaminating the wafer, the porosity of membrane 208 prevents
large size particles from passing through membrane 208. Generally
it is desirable to prevent particles greater in size than one
micron (1.0 .mu.m) from passing through membrane 208.
Virtual anode 10 extends between and is attached on its entire
outer periphery to wall 198 of plating bath 42. In the embodiment
illustrated in FIG. 2, virtual anode 10 has a curved cross-section,
being thinnest at the edge (periphery) and increasing in thickness
toward the center. Virtual anode 10 is provided with a plurality of
openings 10a-10i extending through virtual anode 10 from the bottom
side (the side facing anode cup 202) to the upper side. Openings
10a-10i each have a different length, opening 10e in the center of
virtual anode 10 being the longest and openings 10d-10a and
openings 10f-10i being of gradually reduced length as illustrated.
Further, opening 10e in the center of virtual anode 10 has the
largest radius, while openings 10c, 10d and openings 10f, 10g have
a smaller radius, and openings 10a, 10b and openings 10h, 10i have
an even smaller radius. In the embodiment of FIG. 2, openings 10d,
10c and openings 10f and 10g have equal radii, while openings 10b,
10a and openings 10h, 10i have radii which are smaller than the
remainder of the openings but are equal to each other. However,
this is a matter of choice, the important point being that the
openings control both the electric current flux and the plating
solution flow through virtual anode 10.
Representative dimensions for a typical plating apparatus in
accordance with FIG. 2 are given in Table 1.
TABLE 1 Characteristic Dimension X 8.0 In. Y 9.0 In. Z 10.0 In. A
1.0 In. B 1.0 In. C 1.0 In. D 1.5 In. E 4.89 In. F 7.05 In.
FIG. 3 diagrammatically illustrates one example of the action of
cylindrical openings in a virtual anode in modifying the electric
current flux and the plating solution flow through the virtual
anode. An electric current flux represented by flux lines F is
established between anode 62B and wafer 38, and this electric
current flux is uniform in the immediate vicinity of anode 62B.
However, the presence of virtual anode 100A between anode 62B and
wafer 38 modifies both the electric current flux and the plating
solution flow. The effect on the electric current flux of the
length of the openings in the virtual anode may be likened to a
variable resistance, the longer the path through the virtual anode,
the greater the electrical "resistance" to the electric current
flux. More particularly, the change in electric current flux
through any particular opening is inversely proportional to the
length of the opening. This is illustrated in FIG. 3 where openings
100b and 100c are longer than openings 100a and 100d and thus
present more electrical resistance than do openings 100a, 100d.
Hence, more electric current flux (i.e. a greater percentage of the
total electric current flux to wafer 38) and more flux lines F pass
through the shorter openings 100a and 100d than pass through the
longer openings 100b and 100c resulting in a greater thickness of
the deposited electrically conductive layer on the wafer edge
region. (A greater electric current flux to a particular wafer
region results in a greater thickness of the deposited electrically
conductive layer at that region.)
The plating solution flow through any particular opening is also
inversely proportional to the length of the opening. Thus, although
openings 100a-100d of FIG. 3 have equal radii, the greater length
of openings 100b, 100c will reduce the plating solution flow
therethrough compared to openings 100a and 100d.
For purposes of illustration assume the case where openings 100b
and 100c are twice the length of openings 100a and 100d.
Accordingly, there will be twice the electric current flux and
twice the plating solution flow through openings 100a and 100d
compared to openings 100b and 100c. Thus, a change in the length of
an opening causes a linear change in both the electric current flux
and plating solution flow through the opening. Accordingly a change
in length of an opening does not decouple the electric current flux
from the plating solution flow.
FIG. 4 diagrammatically illustrates another example of the action
of cylindrical openings in a virtual anode in modifying the
electric current flux and plating solution flow through the virtual
anode and, more particularly, in decoupling the electric current
flux from the plating solution flow. In FIG. 4, all openings
100e-100h have equal length, but openings 100e and 100h have a
greater radius than openings 100f and 100g. The electric current
flux through any particular opening is directly proportional to the
square of the radius of the opening. However, the plating solution
flow through any particular opening is directly proportional to the
cube of the radius of the opening. Thus, plating solution flow will
be significantly greater through openings 100e and 100h compared to
openings 100f and 100g. The electric current flux, represented by
flux lines F, will also be greater through openings 100e and 100h
compared to openings 100f and 100g, although to a lesser extent
than plating solution flow. Thus, the percentage of the total
plating solution flow to wafer 38 is significantly greater through
openings 100e and 100h compared to the smaller radius openings 100f
and 100g while the percentage of the total electric current flux to
wafer 38 is only somewhat greater through openings 100e and 100h
compared to the smaller radius openings 100f and 100g.
Since a change in the radius of an opening produces a non-linear
change in the electric current flux compared to the plating
solution flow through the opening, to decouple the electric current
flux from the plating solution flow, the radii of the openings are
adjusted. In one embodiment, by using a plurality of small radius
openings in contrast to a lesser number of larger radius openings,
the total cross-sectional areas of the small radius openings and
the larger radius openings being the same, the plating solution
flow is restricted while the electric current flux remains
essentially unchanged through the openings.
FIG. 5 illustrates an alternate embodiment of a virtual anode
involving a stepped cross-section rather than the contoured
cross-section of the virtual anode of FIG. 2. Virtual anode 10A has
a plurality of openings therein 10j-10r which are generally similar
in configuration and location to openings 10a-10i in the embodiment
of FIG. 2. The only difference between the two embodiments is that,
for ease of fabrication, virtual anode 10A is of a stepped
construction. The operation of the embodiment of FIG. 5 is similar
to that described above for FIG. 2, with the variable lengths and
variable radius of openings 10j-10r controlling the electric
current flux and the plating solution flow through virtual anode
10A. The dimensions given in Table I for the embodiment of FIG. 2
generally apply to the embodiment of FIG. 5.
Although the embodiment of FIG. 2 and FIG. 5 both illustrate
virtual anodes which restrict the plating solution flow to the
wafer edge region compared to the center region while providing a
relatively uniform electric current flux to the wafer plating
surface, it will be apparent that other embodiments of the
invention are possible, including configurations which reduce the
electric current flux and plating solution flow to the central
region of the wafer compared to the edge region, as shown in FIG.
7.
FIG. 6 diagrammatically illustrates another alternate embodiment of
the invention in which the virtual anode 250 takes the form of an
annulus extending inwardly from the top of wall section 218 of
anode cup 202. Virtual anode 250 is a suitable electrical
insulating material and acts as a shield for the flux lines F
emanating through membrane 208 reducing the thickness of the
deposited electrically conductive layer on the edge region of wafer
38. Important dimensions are illustrated in FIG. 6 and include the
distance D between virtual anode 250 and wafer 38, the distance R
which virtual anode 250 extends inward from anode cup 202, and the
distance S representing the spacing between virtual anode 250 and
membrane 208. Generally, the greater distance R is, and the smaller
distances D, S are, the greater the shielding of the wafer edge
region by virtual anode 250. Since each of these dimensions affects
the flux lines F reaching wafer 38 and hence the thickness profile
of the deposited electrically conductive layer, the thickness
profile can be readily adjusted to suit the particular application
by adjusting these dimensions.
FIG. 7 illustrates a further embodiment of the invention which is
adapted for use where it is desired to have less deposited on the
center region of the wafer. In that situation, virtual anode 260
takes the form of a disk of a suitable insulating material which
overlies the center of anode 62A. Virtual anode 260 is suspended by
rib-like members 261 which may be attached to anode cup 202 and
overlie membrane 208. Virtual anode 260 effectively blocks the
electric current flux and plating solution flow to the center
region of the wafer, thereby reducing the thickness of the
deposited electrically conductive layer at the center region of the
wafer. In an alternative embodiment (not shown), a jet or tube is
passed through the center of anode 62A and through the center of
virtual anode 260 to direct plating solution at the center region
of the wafer as further described in Reid et al., application Ser.
No. 08/969,196, cited above, now abandoned.
Having thus described the preferred embodiments, persons skilled in
the art will recognize that changes may be made in form and detail
without departing from the spirit and scope of the invention. Thus
the invention is limited only by the following claims.
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